The invention relates generally to operational amplifier circuits, and more particularly, to balancing circuits used in connection with inverting operational amplifiers for compensating or balancing offset voltages exhibited by the amplifier.
Virtually all operational amplifiers have an inherent offset voltage so that, without an offset voltage balance circuit, the amplifier output voltage is non-zero when the input signal voltage is zero. Thus, an offset balance circuit is required so that an operational amplifier provides a linear amplification of the input voltage signal.
Heretofore, an offset voltage balance has been provided by connecting a balance circuit to apply an adjustable voltage to the non-inverting input of the inverting operational amplifier. Such balance circuits are well known and operate to provide a null or balance so that the amplifier output voltage is zero when the signal voltage to be amplified is zero. Further background and examples of prior balance circuits are contained on pages 114-147 of a book entitle THE VERSATITLE OP AMP written by Michael Kahn, published in 1970 by Holt, Reinhart and Winston, Inc., and assigned Library of Congress Catalog Card Number 77-107333.
Another balancing arrangement provides connecting points from the operational amplifier circuit. The user may attach potentiometers to these connecting points, which are not directly connected to the amplifier inputs, and adjsts the potentiometers to null or balance the offset voltage. A typical operational amplifier having such an offset voltage null capability is designated "micro A741" and is manufactured by Analog Devices Inc., located in Norwood, Mass.
Another balance circuit arrangement for an operational amplifier utilized as a differential amplifier, and connected in circuit to operate as a three-wire transducer, is disclosed in Raymond, U.S. Pat. No. 3,602,832, issued Aug. 31, 1971.
It is well known that in order to provide an inverting operational amplifier circuit having a stable gain, such a circuit should include an input resistor and feedback means such as a feedback resistor. In prior balance circuit arrangements, a change in the resistance value of the input resistor or the feedback resistor has upset the offset voltage balance. This upset in offset voltage has, in many applications, prevented or discouraged the use of relatively inexpensive operational amplifiers, because such amplifiers generally require significantly large input bias currents, and hence, require a new offset voltage balance with every significant change in the resistance values of the input and feedback resistors. More specifically, relatively inexpensive inverting operational amplifiers made up of, for example, a bipolar transistor amplifiers, which require significantly large input bias currents, have heretofore not been able to be effectively utilized in critical applications such as analog computation and various instrumentation circuits for which the input or feedback resistors are frequently changed.
This invention is concerned with the general area and has among its objects to provide a balance circuit which eliminates offset voltage balance problems exhibited by the prior art. More specifically, the invention has among its objects to provide a method and a dual-adjustment balance circuit for operational amplifiers which can be adjusted to achieve a stable offset voltage balance which is generally insensitive to changes in the input and feedback resistors so that relatively inexpensive operational amplifiers can be effectively utilized in critical applications such as analog computation.